Currently the script in dev/merge_arrow_pr.py requires modifying to run on any machines/environments where the shebang line #!/usr/bin/env python finds the system Python 2 instead of Python 3.
Updating the shebang line to read python3 instead of python would fix this.
thisisnic did the same for the linting and formatting scripts in
ARROW-13759. Perhaps it is not as important to do this for the merge script because there is a wrapper script dev/merge_arrow_pr.sh that creates a virtual env and runs the Python script in it, but some Arrow devs (I for one) call the Python script directly.